1. The Field of the Invention
The present invention relates to x-ray devices. More particularly, the present invention relates to x-ray tubes having components constructed of a copper chromium alloy material for enhanced thermal management and thermal stability.
2. The Relevant Technology
X-ray devices are widely used in applications ranging from medical radiology to industrial diagnostics. A common problem encountered in the design and operation of x-ray tubes used in such devices relates to the management of the extremely high temperatures that are present. Heat management is particularly troublesome in the region of the rotating anode and rotor assembly. During operation, extreme temperatures are generated at the anode""s focal track, which are then transferred to other parts of the rotor assembly. These temperatures can adversely affect the operating life of the x-ray tube. For instance, bearings that assist the rotating anode to rotate can fail, and other parts of the rotor assembly are prone to failure from the constant thermal expansion and contraction.
The problems related to high temperatures produced in the x-ray tube have been partly addressed by providing an emissive coating on the rotor portion of the x-ray tube assembly. Preferably, the coating possessed thermal characteristics that allowed components of the x-ray tubexe2x80x94such as the rotorxe2x80x94to operate more satisfactorily under the extreme operating temperatures. For example, in the past a thin, oxygen-deficient titanium oxide layer was applied onto the rotor skirt by a plasma spraying process. However, this coating has not been entirely satisfactoryxe2x80x94especially over longer operating periods. In particular, the repeated thermal cycling of an x-ray tube structure tends to cause an emissive coating of this sort to flake or spall away from the rotor skirt. This debris can then contaminate other components within the x-ray tube, and lead to premature failure of the tube. Moreover, there often is a thermal mismatch between rotor material and the coating material, which tends to weaken the bond between the two materials as they thermally expand. Again, this leads to the undesired situation of the coating flaking or spalling and contaminating the x-ray tube.
Use of such coatings can also give rise to other problems. For instance, during the manufacturing of the x-ray tube device, difficulties are often encountered in getting the coating to properly adhere to the rotor substrate and/or the other x-ray components. To ensure proper adhesion typically requires an additional manufacturing step prepare the rotor substrate. For example, the rotor substrate may be xe2x80x9croughenedxe2x80x9d by blasting the substrate with a grit material. This process is undesirable for several reasons. First, the need for an extra manufacturing step adds cost and complexity to the overall manufacturing processes. Second, some of the grit material used in the roughening process invariably will become physically embedded within the rotor substrate material. This grit material can then shed from the rotor during operation of the x-ray tube, especially after repeated use. Again, release of such foreign matter within the sealed environment of an x-ray tube leads to contamination and premature failure of the tube.
As noted, other components within the x-ray tube are also subject to various problems associated with the high operating temperatures. For example, a bearing support structure is often connected to a xe2x80x9cnosexe2x80x9d portion of the rotor which is in turn connected to the rotating anode. The bearing support structure is typically disposed within the rotor sleeve portion and, due to its close proximity to the rotating anode, is also exposed to extreme temperature fluctuations. Typically, the bearing support structure is made of a copper material to take advantage of its high thermal conductivity qualities. However, copper can deform under the significant transient thermal stress that is experienced in the bearing support structure. A deformed rotor bearing support structure causes problems such as hindered and/or unbalanced rotation, resulting in a cathode-anode misalignment. This situation compromises the quality of the x-rays that are emitted from the anode. Moreover, any type of unbalanced rotation results in vibration of the x-ray tube, which increases operating noise of the x-ray device, and ultimately can render the x-ray tube inoperable.
One approach to address some of the problems encountered when using copper as a rotor bearing support material as been use a alternative material, such as stainless steel. However, although stainless steel exhibits better structural rigidity in the presence of high temperatures and thus resists deformation, stainless steel has a lower thermal conductivity. As such, unacceptably high temperatures can be present within the rotor and bearing assembly.
Another significant challenge for heat management in an x-ray device relates to the dissipation of the heat from the x-ray tube to the surrounding structure. Typically, heat is transferred from the x-ray tube to a heat-transfer fluid medium such as a dielectric oil that is disposed within another enclosure, sometimes referred to as an x-ray tube xe2x80x9ccanxe2x80x9d or housing. This housing or can must also exhibit suitable heat transfer characteristics. If the can is not an efficient heat transfer medium, any efficiencies or improvements achieved for heat transfer in the x-ray tube can be neutralized by the can itself.
Typically, the can housing is made of copper or stainless steel. During operation of the x-ray tube, high temperatures are especially prevalent at the window area of the can, which is where the x-ray signals are emitted. Problems can arise in the event that the material that is adjacent to the window does not efficiently draw heat away from the window.
Thus, what is needed in the art is an x-ray tube that can withstand the destructive effects of extreme operating temperatures generated at the rotating target anode. In particular, the x-ray tube components located adjacent to the anode, such as the rotor and rotor skirt, should possess desirable thermal characteristics. Moreover, any solution should reduce or eliminate the occurrence of any foreign debris being released within the evacuated enclosure, such as from flaking or spalling of any coating materials, or from any materials used during the manufacturing process. In addition, it would be an advancement in the art to provide a x-ray tube housing or xe2x80x9ccanxe2x80x9d that is not subject to warpage and structural damage in the presence of high temperatures, and which can efficiently dissipate heat present at the window area.
It is therefore a primary object of the present invention to provide an x-ray device and manufacturing method in which alloys having superior operating characteristics in the presence of extreme temperatures and temperature fluctuations are utilized.
A related objective of the present invention is to provide a material and a method of manufacture that can be used to construct components of an x-ray tube device and that improves the thermal characteristics of the components.
Yet another objective of the present invention is to provide an x-ray tube having components that are not subject to flaking and spalling of the outer surface, even when exposed to the extremely high operating temperatures within an x-ray tube.
Another object of the present invention is to provide components for use in an x-ray tube that can be manufactured without introducing any foreign debris, such as grit, into the x-ray tube.
Still another objective of the present invention is to provide an x-ray tube that has components that have an outer thermal emitter layer or coating that possesses superior thermal characteristics.
Another related objective is to provide an x-ray tube and method of manufacture in which an outer thermal coating or layer is easily applied to components of the x-ray tube.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. Briefly summarized, the present invention utilizes a chromium alloy of copper material to construct components of an x-ray device that require superior thermal characteristics. For instance, in presently preferred embodiments, the chromium alloy of copper material is used to construct the rotor sleeve, or xe2x80x9cskirtxe2x80x9d portion of the rotor assembly. The rotor sleeve is then treated, for instance in a wet H2 environment, so that the sleeve has an oxidized, or xe2x80x9cgreenedxe2x80x9d external surface. The resulting rotor sleeve exhibits several desirable characteristics. First, the chromium alloy of copper material is structurally sound, even when exposed to the extremely high temperatures of an operating x-ray tube. Second, the outer layer provided by the greened surface is efficient thermal emitter and thus transfers heat away from the rotor assembly. Both advantages result in a rotor assembly that has a longer operational life.
A rotor sleeve constructed of the chromium alloy of copper material also resists any flaking or spalling of the outer surface due to the integral structure of the sleeve and its greened emitter surface. This reduces the amount of contaminant that is present within the evacuated x-ray tube, thereby reducing opportunity for tube failure and increasing the overall operational life of the tube.
Alternative embodiments of the present invention also are directed to a composite structure that uses the inventive chromium alloy of copper as a plasma-sprayed coating upon an essentially oxygen free copper substrate. The plasma-sprayed chromium alloy of copper is then greened in a wet H2 environment. This provides an essentially oxygen free substrate, a copper alloy coating disposed upon the substrate comprising the inventive chromium alloy of copper, and a thermal oxidation layer formed upon the coating. Again, x-ray tube components, such as a rotor sleeve, having this configuration possess superior thermal characteristics.
Other embodiments utilize the chromium alloy of copper in other x-ray tube components. For example, the material can be used in the bearing support structure that is used to provide rotation to the anode disk. Again, the resulting bearing support structure possesses a significantly improved resistance to thermal deformation, and at the same time maintains an efficient high thermal conductivity. This reduces the amount of heat that is conducted to the ball bearing assembly, and reduces the incidence of heat-induced failure that may otherwise occur.
Other embodiments of the present invention relate to the application of the chromium alloy of copper to the outer x-ray tube housing or xe2x80x9ccanxe2x80x9d portion of the x-ray tube device. For example, in one embodiment, the alloy is used within the window frame insert of a stainless steel can housing. This window frame insert acts as an efficient heat sink that draws heat away from the x-ray window itself. Again, this thermal characteristic results in a longer lasting x-ray tube device.
The chromium alloy of copper can also be used in other areas of the x-ray tube can housing. In one preferred embodiment, the entire can is constructed of the chromium alloy of copper, and is then treated so as to provide the outer thermal oxidation layer. Again, the resulting can provides distinct advantages of thermal heat management and resistance to deformation.
Embodiments of the present invention also relate to an improved method of making the inventive chromium alloy of copper. One preferred method includes providing essentially oxygen free copper as a major component with unavoidable impurities and combining it with chromium as a minor component with unavoidable impurities. The combination of copper and chromium is placed into a container in an inert atmosphere and heated in order to achieve a chromium copper solution. Embodiments of the present invention also involve methods of using the chromium alloy of copper, including the casting of articles of manufacture therefrom and also atomizing the chromium copper solution to obtain a metal powder as a stock material for forming preferred articles by powder metallurgical techniques.